The use of discrete fibers in the reinforcement of concrete is set forth in U.S. Pat. No. 3,645,961. The patent discloses the use of nylon, polyvinyl chloride and simple polyolefins in inches ranging between one-quarter to three inches (0.6 to 7.5 cm) to form a blast resistant concrete.
U.S. Pat. No. 4,306,911 teaches that water sensitive and water-insensitive fibers can be treated with materials that coat both types with a water-insoluble material. The resulting combination of filter fibers and reinforcing fibers can be used to produce asbestos-free fiber-reinforced cement products with similar product and performance properties.
British 1,406,442 discloses that reinforced concrete articles which contain fiber strands of at least two fibers twisted together have better tensile strength, crack control, impact resistance, toughness and energy absorption when subjected to destructive (e.g. explosive) loads. The result cannot be achieved merely be adding the strands to conventional concrete mixes, but require adjustments in coarse and fine aggregate volumes, and the inclusion of significant proportions, such as ten percent, of fines, such as pulverized fuel ash. A mixing aid, such a polyethylene oxide, is also recommended to enable increased amounts of fibers to be incorporated. Improvements in strengths depend upon a judicious combination of different degrees of length, cross-sectional dimensions and proportions of mixed fibers, leading to multiple cracking.
The use of fibrous materials made from nylon, polypropylene, polyvinylidene chloride and polyethylene is set forth in U.S. Pat. No. 3,645,961. Less than 3 percent of these fibers in lengths from 1/4 to 3 inches (0.6 to 7.5 cm) can be mixed into concrete to make blast-resistant structures.
The use of fibrillaced polypropylene fibers from 0.05 to 2 percent by weight of the total wet mixture of water-hardenable inorganic materials is presented in U.S. Pat. No. 3,591,395.
Different fibers and polymer materials have been added to concrete and cement for a variety of purposes. U.S. Pat. No. 3,056,686 for instance, is directed toward a mass of glass reinforcement and a binder comprising an inorganic silicate. The invention resides in the binder, which is alkaline, but is reacted with an ion exchange material to prevent it from attacking the fiber glass reinforcement.
U.S. Pat. No. 2,793,130 provides a method for protecting glass fiber reinforcement in cement, which involves coating the glass fibers prior to contact with the cement. The coating comprises a water-soluble salt of a polyvalent metal, applied to the fibers, which is then treated with a material that can release hydroxy ions. The latter react in situ on the glass to form a water-insoluble reaction products with the soluble salt.
These patent references represent the significant prior art on the use of fibers in concrete over the last thirty-five years. This body of art addresses the use of fibers, alone or in combination with other materials, that perform primary reinforcing functions. The present invention addresses the issue of secondary reinforcement, as it applies to the prevention of shrinkage cracking and the behavior of the fibers covered by the invention after cracking under flexural stress.
Despite the fact that fibers and other materials have found great utility as additions to cement, concrete, precast products, mortar, stucco and the like, as secondary reinforcements, it has not been known heretofore to employ a plurality of fiber designs designed to an optimum gradation in one mixture for incorporation into concrete and like materials.